(a) Field of the Invention
The present invention relates to a martensitic stainless steel, and more particularly, to a martensitic stainless steel that can be used in manufacturing articles such as a shaft or an impeller or a knife which require high mechanical strength and corrosion resistance.
(b) Description of the Related Art
Generally, stainless steel is classified according to its main element, as martensitic stainless steel, austenitic stainless steel, ferritic stainless steel, duplex stainless steel, precipitation hardening stainless steel, and superstainless steel.
Such stainless steels guarantee necessary mechanical strength and corrosion resistance. Therefore, they are widely used to manufacture industrial equipment and parts thereof requiring such characteristics.
Particularly, the well-known 410 or 420-martensitic stainless steel has high mechanical strength, being used as a material for manufacturing boiler pump impellers that are subject to high centrifugal force or pipes for delivering fluid containing abrasive particles.
A recently developed power plant has been designed such that the inlet temperature of a boiler turbine is increased to improve fuel efficiency. Furthermore, the rotational speed of the turbine pump is also increased. Therefore, there is a need for a material which can endure the rigorous conditions of the power plant.
Particularly, components of a boiler feed pump should have high corrosion and wear resistance as well as high mechanical strength, since it must convey fluid containing oxide scale or fine solid particles such as, coal or sand which cause abrasion of the components.
However, the 410 and 420J-martensitic stainless steels, which have been used to manufacture components of the conventional boiler feed pump and have high mechanical strength, have a disadvantage in that they have a relauvely low corrosion resistance.
That is, since the martensitic stainless steel contains a relatively high carbon weight and 12-18% chrome, the mechanical strength thereof can be increased to a high level through a heat-treatment process. But since chrome carbide is precipitated on the grain boundary due to the high carbon content, a chrome-exhausted layer is easily formed, deteriorating the corrosion resistance.
A supermartensitic 248SV alloy developed by Avesta Sheffield Company is well known as a material used for manufacturing the boiler feed pump. This alloy is improved in terms of corrosion resistance by reducing the carbon content, increasing the chrome content and adding nickel. However, since it is difficult in the alloy to have high percentage of martensitic structure owing to the adding nickel which is one of austenite stabilizing element, the alloy is deteriorated in terms of mechanical strength.
Japanese Patent No. JP-61030665 discloses a stainless steel which is improved in terms of corrosion resistance with respect to seawater and other fluids. However, since the stainless steel contains 3.5-7.0% cobalt, the manufacturing cost is increased. Furthermore, more than 0.08% carbon content deteriorates the corrosion resistance.
Therefore, there is a need for a stainless steel which has both high mechanical strength and high corrosion resistance such that it can be optimally used for manufacturing a boiler feed pump impeller and shaft.
The present invention has been made in an effort to meet the above need.
To meet the above need, the present invention provides a martensitic stainless steel comprises less than 0.06 wt. % C, less than 2.5 wt. % Si, less than 2.5 wt. % Mn, 1.0-6.0 wt. % Ni, 10.0-19.0 wt. % Cr, 0.5-6.0 wt. % W, less than 3.5 wt. % Mo, less than 0.8 wt. % Nb, less than 0.8 wt. % V, less than 3.0 wt. % Cu, 0.04-0.25 wt. % N, and the remainder being Fe and minor impurities.
Preferably, the martensitic stainless steel may further comprise at least one of less than 0.8 wt. % Ti and/or 1.0 wt. % Ta.
According to another aspect of the present invention, a martensitic stainless steel comprises less than 0.035 wt. % C, less than 2.0 wt. % Si, less than 2.0 wt. % Mn, 1.5-4.5 wt. % Ni, 12.0-16.0 Wt. % Cr, 0.5-4.5 wt. % W, less than 2.5 wt. % Mo, less than 0.3 wt. % Nb, less than 0.3 wt. % V, less than 2.0 wt. % Cu, 0.08-0.20 wt. % N, and the remainder being Fe and minor impurities.
According to another aspect of the present invention, a method for manufacturing a martensitic stainless steel comprises the steps of casting a stainless steel comprises less than 0.06 wt. % C, less than 2.5 wt. % Si, less than 2.5 wt. % Mn, 1.0-6.0 wt. % Ni, 10.0-19.0 wt. % Cr, 0.5-6.0 wt. % W, less than 3.5 wt. % Mo, less than 0.8 wt. % Nb, less than 0.8 wt. % V, less than 3.0 wt. % Cu. 0.04-0.25 wt. % N, and remainder being Fe and minor impurities; and submitting the cast stainless steel to an austenization heat treatment at a temperature of 800-1150xc2x0 C. and/or tempering the stainless steel at a temperature of 350-575xc2x0 C.
According to still another aspect of the present invention, a method for manufacturing a martensitic stainless steel comprises the steps of casting a stainless steel comprises less than 0.06 wt. % C, less than 2.5 wt. % Si, less than 2.5 wt. % Mn, 1.0-6.0 wt. % Ni, 10.0-19.0 wt. % Cr, 0.5-6.0 wt. % W, less than 3.5 wt. % Mo, less than 0.8 wt. % Nb, less than 0.8 wt. % V, less than 3.0 wt. % Cu, 0.04-0.25 wt. % N, and remainder being Fe and minor impurities; mechanical-processing the stainless steel such that work hardening is generated in the stainless steel; and submitting the mechanical-processed stainless steel to austenization heat treatment at a temperature of 800-1150xc2x0 C. and/or tempering the stainless steel at a temperature of 350-575xc2x0 C.